On a sliding gate, the catcher post (also 'receive post' or 'closing post') sits at the opposite end of the opening from the runout. It does three things at once. It physically stops the gate at its closed position. It guides the leading edge laterally so the gate doesn't kick sideways. It usually carries the latch or lock target.
A catcher post for a residential sliding gate is typically a 100x100x4 SHS in concreted footing. For commercial gates and longer leaves it goes up to 125x125x5 or 150x150x6 to handle the moment from a kicked or wind-loaded gate face.
Design-wise, the catcher post is also where the stop block, leading-edge latch, and any safety photo-eye get mounted, so it carries more than just the gate impact load.